Diagnose faults in heavy goods and public service vehicles where no prescribed process or format is available

Business Sectors (Suites): Maintenance and Repair - Heavy Goods and Public Service Vehicles
Developed by: IMI
Approved on: 30 Mar 2021


This standard is about devising and implementing strategies to diagnose faults on heavy goods and public service vehicles when the application of standard manufacturer diagnostic procedures has failed to reveal the source and cause of problems.  You are also required to identify the best course of action to be taken to correct problems.

Performance criteria

You must be able to:

P1 use suitable personal and vehicle protective equipment throughout all diagnostic related activities in the workshop
P2 confirm with the relevant people that all standard diagnostic procedures and techniques have been systematically and correctly applied to the vehicle prior to undertaking further work
P3 analyse all previous system fault information, diagnostic test methods and results correctly to verify the inconclusive results prior to undertaking further work
P4 liaise with the relevant manufacturer's representative to obtain up to date information, advice and guidance relevant to the identified fault, where necessary
P5 use diagnostic methods which are relevant to the symptoms presented
P6 collect diagnostic information in a systematic and structured way which progressively eliminates all possible causes of the fault
P7 apply the checks and tests that are most likely to be effective in revealing the cause of the fault
P8 carry out all diagnostic activities following:

P8.1 your workplace procedures
P8.2 health, safety and environmental requirements
P9 work in a way which minimises the risk of:

P9.1 damage to other vehicle systems, units and components
P9.2 contact with leakages and hazardous substances
P9.3 damage to your working environment
P9.4 injury to self and others
P10 use any equipment required, correctly and safely throughout all diagnostic and rectification activities
P11 collect sufficient diagnostic information to enable an accurate diagnosis of the fault 
P12 correctly identify the cause(s) of the fault
P13 identify and record any system deviation from acceptable limits 
P14 ensure your assessment of dismantled sub-assemblies, units and components identifies their condition and suitability for repair or replacement
P15 make clear recommendations for a suitable course of action to rectify the fault
P16 promptly inform the relevant person(s) where repairs are uneconomic or unsatisfactory to perform
P17 complete all system checks and tests in the most cost and time effective way for the fault presented
P18 complete all system diagnostic activities within the agreed timescale
P19 ensure your records are accurate, complete and passed to the relevant person(s) within the agreed timescale in the format required
P20 promptly report any anticipated delays in completion to the relevant person(s)

Knowledge and Understanding

You need to know and understand:

Legislative and organisational requirements and procedures
K1 the legislation and workplace procedures relevant to: 

K1.1 health and safety
K1.2 the environment (including waste disposal)
K1.3 appropriate personal and vehicle protective equipment
K2 the legal requirements relating to the vehicle (including road safety requirements)
K3 the implications on an Operators Licence of not carrying out repairs correctly
K4 how to formulate and construct your own diagnostic procedures and processes in order for diagnostic activities to proceed
K5 the importance of documenting diagnostic and rectification information following your workplace procedure
K6 the importance of working to agreed timescales and keeping others informed of progress
K7 the relationship between time, costs and productivity
K8 your workplace procedure for the referral of problems
K9 the importance of promptly reporting anticipated delays to the relevant person(s) following your workplace procedure

Electrical and electronic systems
K10 the hazards associated with working on or near high voltage electrical vehicle components
K11 electrical and electronic systems including types of sensors and actuators, their application and operation
K12 how electrical and electronic vehicle systems operate, including electrical component function, electrical inputs, outputs, voltages and oscilloscope patterns, digital and fibre optics systems
K13 the interaction between electrical, electronic, mechanical, pneumatic and hydraulic components within a vehicle
K14 how mechanical, hydraulic and electrical systems interlink and interact, including multiplexing
K15 electrical symbols, units and terms
K16 electrical safety procedures 


Use of diagnostic and rectification equipment
K17 how to prepare and check the accuracy of *diagnostic testing equipment
K18 how to use diagnostic and rectification equipment for mechanical, electrical, electronic, pneumatic, hydraulic and fluid systems, specialist repair tools and general workshop equipment

Vehicle system faults, their diagnosis and correction
K19 how vehicle mechanical, electrical, electronic, pneumatic, hydraulic and fluid systems are constructed, dismantled, reassembled and operate
K20 the types and causes of vehicle mechanical, electrical, electronic, pneumatic, hydraulic and fluid system, component and unit faults and failures
K21 vehicle mechanical, electrical, electronic, pneumatic, hydraulic and fluid component and unit replacement procedures, the circumstances which will necessitate replacement and other possible courses of action
K22 how to find, interpret and use sources of information on vehicle mechanical, electrical, electronic, pneumatic, hydraulic and fluid system specifications, diagnostic test procedures, repair procedures and legal requirements 
K23 vehicle operating specifications for limits, fits and tolerances relating to vehicle mechanical, electrical, electronic, pneumatic, hydraulic and fluid systems for the types of vehicle on which you work
K24 how to select the most appropriate diagnostic testing method for the symptoms presented
K25 how to carry out systematic diagnostic testing of vehicle mechanical, electrical, electronic, pneumatic, hydraulic and fluid systems
K26 how to assess the condition of the systems and components within vehicle mechanical, electrical, electronic, pneumatic, hydraulic and fluid systems
K27 how to interpret, evaluate and analyse test results and vehicle data in order to identify the location and cause of vehicle system faults
K28 how to carry out the rectification activities in order to correct faults in the vehicle mechanical, electrical, electronic, pneumatic, hydraulic and fluid systems

K29 your workplace procedure and policy for:

K29.1 work carried out under warranty
K29.2 liaising with manufacturers and outside agencies
K30 the relationship between test methodology and the faults repaired – the use of appropriate testing methods
K31 how to make cost effective recommendations for rectification
K32 the importance of inspecting the vehicle following any repairs


1. Causes of the fault are:

1.1. mechanical
1.2. electrical
1.3. electronic
1.4. hydraulic
1.5. pneumatic

2. Faults cover the:

2.1. vehicle engine area
2.2. transmission and driveline area
2.3. chassis system area
2.4. electrical and electronic units and components area

3. Diagnostic methods are:

3.1. sensory
3.2. functional 
3.3. measurement
3.4. electrical and electronic systems testing

4. *Diagnostic testing *is defined as:

4.1. verify the fault
4.2. collect further information
4.3. evaluate the evidence
4.4. carry out further tests in a logical sequence
4.5. rectify the problem
4.6. check all systems

5. Equipment is:

5.1. diagnostic and rectification equipment for mechanical systems
5.2. diagnostic and rectification equipment for electrical and electronic systems
5.3. diagnostic and rectification equipment for hydraulic and fluid systems
5.4. diagnostic and rectification equipment for pneumatic systems
5.5. specialist repair tools
5.6. general workshop equipment​

Scope Performance

Scope Knowledge





This section contains examples and explanations of some of the terms used but does not form part of the standard.

Heavy goods and public service vehicles
These are medium and large goods vehicles, buses and coaches of 3500kgs gross vehicle mass (GVM) and above.

Rectification activities
These are a suitable repair, replacement, re-coding or re-programming that rectifies the fault(s) identified form the diagnostic activities carried out.

Sensory testing methods
These may include looking, listening, smelling and touching for heat.

Links To Other NOS

External Links

Version Number


Indicative Review Date

31 Mar 2024





Originating Organisation


Original URN


Relevant Occupations

Engineering, Heavy Vehicle Diagnostic Technician (Automotive), Heavy Vehicle Master Technician (Automotive), Vehicle Trades, Bus and Coach Mechanic, Bus and Coach Mechelec

SOC Code



Diagnose; faults; heavy goods vehicle; public service vehicle; prescribed; guided; systematic; bus; coach;